1. Field of the Invention
The present invention relates to a battery power detection device, and in particular, to a battery power detection device for detecting remaining power of a secondary battery used in a mobile phone or other portable apparatuses.
2. Description of the Related Art
In a portable apparatus using a secondary battery as a power supply, such as a mobile phone and a notebook personal computer, usually, a battery power detection device is installed to detect the remaining power (stored energy) of the secondary battery (below, just abbreviated to be “battery power” or “power of battery” where necessary). For example, the detected remaining power of the secondary battery can be presented on a display to notify the user of the time to charge the battery.
In the related art, for example, a battery power detection device detects the battery power by means of current integration or voltage measurement. In the method of current integration, the current supplied to a load from the battery in use is integrated throughout the usage duration, and from the integration result, the used power (expended energy) of the battery is calculated, and the remaining power is obtained by subtracting the used power from the total power (starting power). With this method, the power of the battery can be calculated accurately, however, such a battery power detection device is expensive, and since current integration has to be carried out over all the time when the battery discharges, the power consumed by the battery power detection device is not negligible.
In the method of voltage measurement, the output voltage of the battery is measured with the battery being connected to with a specified load, and the power of the battery is predicted from the measured voltage. With this method, the battery power detection device is operated only when it is necessary to detect the power of the battery; hence, the power consumed by the battery power detection device is low, and the battery power detection device is inexpensive. However, the detection accuracy is low compared to the current integration method.
The method of voltage measurement utilizes the characteristic that the output voltage of the battery decreases along with the decrease of battery power. Since the relationship between the output voltage of the battery and the battery power varies along with the temperature of the battery and the load current of the battery, it is not guaranteed that the battery power can be accurately predicted from the measured output voltage of the battery. In the battery power detection device of the related art which uses the method of voltage measurement, in order to improve the measurement accuracy, the temperature and the load current of the battery are also measured during battery power detection, and the power of the battery is predicted from the measured voltage, temperature, and the load current of the battery.
For example, Japanese Patent Gazette No. 3087276 discloses a battery power detection device in which plural data tables are installed to define the battery voltages of stepwise levels corresponding to different discharging currents of the battery in different temperature sections. This battery power detection device includes a data processing unit, a voltage input unit for inputting the battery voltages to the data processing unit, a current input unit for inputting the discharging currents of the battery to the data processing unit, a temperature input unit for inputting the battery temperatures to the data processing unit, and a display. The data processing unit generates and outputs a signal representing the stepwise level of the battery voltage indicating the battery power with the battery voltage being corrected with a discharging current by using one of the data tables. The display processes the signal output from the data processing unit and displays the signal in a stepwise manner.
In addition, for example, Japanese Laid-Open Patent Application No. 11-162524 discloses a battery power detection device which includes a unit for determining the power of a battery based on output of a voltage sensor and a current sensor, and the power determined by the unit is corrected according to output of a temperature detector for detecting the temperature of the battery. In the unit for determining the battery power, there is stored a data table containing battery power data determined according to the relation between voltage and current of the battery. The unit determines the battery power from the output of the voltage sensor and the current sensor, and decides whether the determined battery power is greater than a specified reference value. In this process, the unit corrects the determined battery power using the temperature of the battery given by the temperature detector arranged near the battery.
However, in the battery power detection devices of the related art, which detect the battery power by voltage measurement, it is required to store a two-dimensional data table in order to determine the battery power based on the relation between the output voltage and the power of the battery, and in addition, in order to make temperature corrections, it is required to stores plural data tables. Due to this, in the battery power detection devices of the related art which detect the battery power by voltage measurement, it is required to install a memory having a sufficiently large capacity to store the data tables.
Even in the battery power detection devices of the related art which detect the battery power by current integration, a memory having a sufficiently large capacity is required in order to perform integration over a long duration.
Consequently, in each of the battery power detection devices of the related art, there exists a memory having a large capacity, which in turn results in a circuit of a large area.